The present invention relates generally to a speed control system for motor vehicles, and more particularly to a control system having a function to adjust a control characteristic of a control unit of an actuator to regulate a speed of a motor vehicle to a target value when the motor vehicle is under cruising control.
The Japanese Utility Model Provisional Publication No. 64-44230 discloses such a conventional speed control system which estimates the dynamic characteristic of a motor vehicle, varying in accordance with the travelling condition of the motor vehicle, in real time so as to obtain the control coefficient for the PID control calculation on the basis of the estimation result. This conventional speed control apparatus can obtain the control characteristic coping with the variation of the operating condition which is a disturbance for the constant-speed (cruising) control operation.
Here, a model of a motor vehicle depends upon the static characteristic and dynamic characteristic of the motor vehicle as shown in FIG. 23. The static characteristic of the motor vehicle represents the relation between an opening degree of a throttle valve and the speed of the motor vehicle steadily obtained in response to the throttle opening degree. The throttle opening degree and a vehicle speed are substantially in the proportional relation to each other, and hence the static characteristic can be approximated with a linear function. On the other hand, the dynamic characteristic of the motor vehicle represents the relation between the variation of the throttle opening degree (input) and the variation of the vehicle speed (output) occurring in response to the variation of the throttle opening degree. This dynamic model of the motor vehicle is the vehicle model to be estimated, and can be expressed with a first-order transfer function if taking into account the weight of the motor vehicle and others. The gain K in the transfer function is indicative of the variation (k/h) of the vehicle speed taken when the throttle opening degree varies by one, and the time constant .tau. is representative of the time taken until the vehicle speed variation reaches 63% of the gain K.
Thus, if assuming that the vehicle static characteristic is fixed, the dynamic characteristic in the vehicle model can be estimated on the basis of the variation of the vehicle speed to the variation of the throttle opening degree detected. Further, the actual vehicle speed estimation value becomes a value obtained by adding the vehicle speed due to the dynamic characteristic component caused by the estimated dynamic characteristic to the vehicle speed due to the static characteristic.
However, for example, when the slope of the road surface, the load of the vehicle or others varies the static characteristic of the motor vehicle varies. In the case of the variation of the vehicle static characteristic, if the dynamic characteristic is estimated merely on the basis of the detected vehicle speed and throttle opening degree, the vehicle speed and the throttle opening degree vary in accordance with the variation of the vehicle static characteristic whereby there is the possibility that the vehicle dynamic characteristic is estimated in error.